Lighting Design Glossary

Color Temperature
Correlated Color Temperature (CCT)
Kelvin

(Term of photometry)

Color temperature is a simplified way to characterize the spectral
properties of a light source. While in reality the color of light
is determined by how much each point on the spectral curve
contributes to its output, the result can still be summarized on
a linear scale.

This value is useful eg. for determining the correct film in
photography depending on the lighting (resp. for determining the
white balance in digital photography), and for specifying the
right light source types in architectural lighting design.
Note, however, that light sources of the same color
(metamers) can vary widely in the quality of light
emitted. One may have a continuous spectrum, while the other just
emits light in a few narrow bands of the spectrum. A useful way
to determine the quality of a light source is its color
rendering index.

Low color temperature implies warmer (more yellow/red) light
while high color temperature implies a colder (more blue) light.
Daylight has a rather low color temperature near dawn, and a
higher one during the day. Therefore it can be useful to install
an electrical lighting system that can supply cooler light to
supplement daylight when needed, and fill in with warmer light at
night. This also correlates with human feelings towards the warm
colors of light coming from candles or an open fireplace at night.

Standard unit for color temperature is Kelvin (K).

(The kelvin unit is the basis of all temperature measurement,
starting with 0 K (= -273.16° C) at the absolute zero
temperature.
The "size" of one kelvin is the same as that of one degree
Celsius, and is defined as the fraction 1/273.16 of the
thermodynamic temperature of the triple point of water, which
positions 0° Celsius at 273.16 K.)

Technically, color temperature refers to the temperature to which
one would have to heat a theoretical "black body" source to
produce light of the same visual color.